Friction vacuum pump with a stator and a rotor

ABSTRACT

The invention relates to a friction vacuum pump ( 1 ) with a stator ( 3 ) and a rotor ( 4 ), which form at least two pump stages ( 12, 13, 14 ) with one gas inlet ( 23, 28 ) each, as well as with junction for the pump stages, which are equipped with junction openings ( 36,37 ) and serve for the connection of the gas inlets ( 23, 28 ) of the pump stages with devices to be evacuated; in order to avoid high conductance losses it is proposed that the junction openings ( 36, 37 ) are located in a plane which is disposed laterally adjacent to the pump stages ( 12, 13, 14 ) such that the distance between the junction openings ( 36, 37 ) and the rotor axis ( 15 ) is of minimum feasible size.

BACKGROUND OF THE INVENTION

The invention relates to a friction vacuum pump with a stator and arotor, which form at least two pump stages with one gas inlet each, aswell as junction means for the pump stages, which are equipped withjunction openings and serve for connecting the gas inlets of the pumpstages with devices to be evacuated.

A friction vacuum pump of this type is known from DE-A-43 31 589. Itserves preferably for evacuating particle beam apparatus (for examplemass spectrometers) with chambers separated from one another bydiaphragms, in which different pressures are to obtain during operationof the particle beam apparatus. It is known per se to use separatevacuum pumps for generating these pressures.

DE-A-43 31 589 discloses generating with the aid of only one vacuum pumpsystem the different pressures required by the particle beam apparatus.The pump system comprises two turbomolecular and one molecular (Holweck)pump stage. These pump stages are disposed such that one axiallysucceeds the other. Each pump stage comprises a gas inlet (front-sidegas penetration area), which, via junction means, is connected with theassociated chamber of the device to be evacuated. In the solutionaccording to DE-A-34 31 589 the housing itself and a laterally disposedauxiliary housing serve as junction means. The housing itself isequipped with a front-side junction opening for connecting the gas inletof the first pump stage with the device to be evacuated. In theauxiliary housing are provided connection lines which connect theassociated inlets of the further pump stages with further junctionopenings. These are each connected, in turn, with the associatedchambers in the device to be evacuated. Since the junction openings inthe auxiliary housing are located in a common plane (perpendicularly tothe rotor axis) with the junction opening of the first pump stage, theconnection lines located in the auxiliary housing, must be relativelylong. Thereby relatively large conductance losses in the connectionlines result, which is in particular of disadvantage if a high suctioncapacity is desired precisely in the region of an intermediate junction.

SUMMARY OF THE INVENTION

The present invention is based on the task of implementing a frictionvacuum pump of the above described type such that the suction capacityof the intermediate stages is not impaired by high conductance losses inconnection lines.

This task is solved according to the invention thereby that the junctionopenings are located in a plane laterally adjacent to the pump stagessuch that the spacing between the junction openings and the rotor axisis of minimum feasible size.

These measures ensure that the spacing between the particular gas inletof the intermediate stages and the associated junction openings is alsoof minimum feasible size. Conductance losses are low. The suctioncapacity active in the region of the gas inlet of all pump stages isavailable nearly unchanged even in the region of the associated junctionopenings.

While realization of the measures according to the invention leads tothe fact that the gases to be transported in the inlet region of thefirst pump stage, thus exactly at that site at which the pressure islowest, must be deviated, however, the loss in conductance causedthereby can be kept low since the spacing between the gas inlet and theplane of the junction opening still is relatively small and, inaddition, nothing stands in the way of selecting in this region agreater diameter. Moreover, for the majority of applications especiallyhigh values for the suction capacity are not demanded in the region ofthe inlet of the first (high-vacuum side) pump stage. There isfrequently even the necessity to reduce the suction capacity at thissite. It is the essential purpose of the first pump stage to ensure ahigh compression ratio. The blade properties (number of turbo stages,blade spacing, angle of inclination etc.) must be designed with this inmind. Essential is the separation of the two working pressure regions ofthe two pump stages. As a rule, high suction capacity is only requiredat the intermediate inlet(s). This goal can also be attained through theselection of special blade geometries. Applying the measures accordingto the invention ensures precisely in this region that losses in suctioncapacity are largely avoided.

Critical for the suction capacity of a pump stage is the accessibilityof the gas molecules to the gas inlet (effective gas penetration area).In order to attain this goal, it is known to provide in an intermediatestage a greater spacing between the preceding stage and its gas inlet.It is especially advantageous if this spacing is at least one fourth,preferably one third, of the diameter of the rotor.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and details of the invention will be explained inconjunction with embodiment examples depicted in FIGS. 1 and 2 wherein:

FIG. 1 is a side elevation in section illustrating a pump embodying theteachings of the present invention; and

FIG. 2 is a side elevation in section illustrating a second embodimentof the invention.

DESCRIPTION OF THE INVENTION

In both Figures the pump itself is denoted by 1, its housing by 2, itsstator system by 3 and its rotor system by 4. The rotor system comprisesthe shaft 5, which, in turn, is supported via the bearings 6, 7 in thebearing housing 8 connected with the pump housing 2. In the bearinghousing is disposed, in addition, the driving motor 9, 10. Therotational axis of the rotor system 4 is denoted by 15.

Overall, three pump stages 12, 13, 14 are provided, of which two (12,13) are developed as turbomolecular vacuum pump stages and one (14) asmolecular (Holweck) pump stage. Adjoining the molecular pump stage 14 isthe outlet of a pump 17.

The first pump stage 12, disposed at the high-vacuum side, comprisesfour pairs of rotor blade rows 21 and stator blade rows 22. Its inlet,the effective gas penetration area is denoted by 23. Adjoining the firstpump stage 12 is the second pump stage 13, which comprises three pairseach of a stator blade row 22 and a rotor blade row 21. Its inlet isdenoted by 28.

The second pump stage 13 is spaced apart from the first pump stage 12.The selected distance (height) a ensures the free accessibility of thegas molecules to be transported to the gas inlet 28. The distance a isusefully greater than one fourth, preferably greater than one third ofthe diameter of the rotor system 4.

The adjoining Holweck pump comprises a rotating cylinder segment 29which is opposed on the outside and inside in known manner by statorelements 32, 33 each equipped with a threaded groove 30, 31.

The rotor-side components of pump stages 12, 13, 14 form a unit which,in the operationally ready state are connected with the shaft 5. At thelevel of the interspace between the pump stages 12 and 13 the shaft 5penetrates a central bore 25 such that no direct connection existsbetween the bearing space and the interspace and, consequently, thedanger of back diffusion of lubricant vapors is eliminated. For thispurpose serves also the taper-bore mounting of the rotor system 4.Bearings disposed at the high-vacuum side with the structural components(bearing supports) impairing conductance can be omitted. However, bydeveloping the portion of the rotor system 4 in the proximity of themotor as a bell-shaped form, the distance of the bearing 6, 7 from thecenter of gravity of the rotor is kept small. The back diffusion oflubricant vapors can also be avoided by using magnet bearings which canbe disposed at a more favorable site.

For the realization of the junction means according to the inventionserves the housing 2 itself. In the embodiment example according to FIG.1 it is developed such that the planes of all junction openings 36, 37are parallel to the rotor axis 15. Thereby in particular the distance ofthe junction 37 to the associated gas inlet 28 is very small such thatthe conductance losses impairing the suction capacity of the pump stage13 are negligible. This would also apply to every further intermediatejunction disposed downstream from the intermediate junction 37/28. Thediameter of the junction opening 37 here exceeds the height a byapproximately the twofold. This measure also serves for decreasing theconductance losses between inlet 28 and junction opening 37.

The depicted pump 1 or its effective pumping elements (stator and rotorblades, threading stages) are usefully developed such that in the regionof the junction opening 36 a pressure is generated of 10⁻⁴ to 10⁻⁷,preferably 10⁻⁵ to 10⁻⁶, and in the region of the junction opening 37 apressure of approximately 10⁻² to 10⁻⁴ mbar. This creates the necessityfor the first pump stage 12 to provide a compression ratio of 10² to10⁴, preferably greater than 100. With the second pump stage a highsuction capacity is to be generated (for example 200 l/s). The adjoiningtwo-stage Holweck pump stage (29, 30; 29, 31) ensures a high fore-vacuumimmunity such that customarily the suction capacity of the second pumpstage is independent of the fore-vacuum pressure.

For the case that in the region of the junction opening 36 an especiallyhigh suction capacity is not required, this goal can be attained throughcorresponding formation of the blades of the first pump stage 12. Afurther feasibility comprises disposing in front of inlet 23 of thefirst pump stage a diaphragm 38 whose inner diameter determines thedesired suction capacity.

The embodiment example according to FIG. 2 differs from the embodimentexample according to FIG. 1 thereby that the diameter of the pump stages13 and 14 succeeding the first pump stage 12 are greater than thediameter of pump stage 12. The plane of the junction openings 36, 37 isadapted to this structural condition. It is inclined with respect to theaxis 15 of rotor 4 such that the distance of the junction openings 36,37 to the associated gas inlets 23, 28 is as small as feasible. Theangle of inclination α of the plane of the junction openings 36, 37 tothe rotor axis 15 corresponds to the increase of the diameters of thepump stages. Optimally favorable distance conditions can thereby beattained. In the embodiment example depicted, the angle of inclinationis approximately 5°.

What is claimed is:
 1. Friction vacuum pump (1) with a stator (3) and arotor (4), which form at least two pump stages (12, 13, 14) with one gasinlet (23, 28) each, as well as with junction means for the pump stageswhich are equipped with junction openings (36, 37) and serve for theconnection of the gas inlets (23, 28) of the pump stages with devices tobe evacuated, characterized in that the junction openings (36, 37) areeach in a plane disposed laterally adjacent to the pump stages (12, 13,14).
 2. Friction vacuum pump as claimed in claim 1, characterized inthat the planes of the junction openings (36, 37) are disposed parallelto the axis (15) of the rotor (4).
 3. Friction vacuum pump as claimed inclaim 1, characterized in that the junction openings (36, 37) areconstituents of the housing (2) of the friction vacuum pump (1). 4.Friction vacuum pump as claimed in claim 1, characterized in that thetwo first pump stages (12, 13) are developed as turbomolecular pumpstages and that their effective pumping elements (stator and rotorblades) are formed such that the first pump stage (12) ensures a highcompression ration and that the second pump stage (13) generates a highsuction capacity.
 5. Friction vacuum pump as claimed in claim 4,characterized in that the two pump stages (12 and 13) are spaced apartand that their distance (a) is greater than one fourth of the rotordiameter, preferably approximately one third of the rotor diameter. 6.Friction vacuum pump as claimed in claim 5, characterized in that thediameter of that junction opening (37), which is connected via thejunction means with the gas inlet (28) of the second pump stage, isgreater than the distance (a), preferably approximately twice as largeas the distance (a).
 7. Friction vacuum pump as claimed in claim 4,characterized in that the two pump stages (12, 13) are adjoined by atwo-stage Holweck pump stage.
 8. Friction vacuum pump as claimed inclaim 1, characterized in that the rotor (4) is driven at thefore-vacuum side and is taper-bore mounted.
 9. Friction vacuum pump asclaimed in claim 8, characterized in that a free shaft end penetrates acentral bore (25) in the rotor (4) and that the rotor (4) is fastened onthis shaft end.
 10. Friction vacuum pump as claimed in claim 8,characterized in that the portion of the rotor (4) in the proximity ofthe motor is developed in the form of a bell.
 11. Friction vacuum pumpas claimed in claim 1, characterized in that with the inlet (23) of thefirst pump stage (12) is associated a diaphragm (38) for limiting thesuction capacity.
 12. Friction vacuum pump as claimed in claim 1,characterized in that it is equipped with magnet bearings.
 13. Afriction vacuum pump as set forth in claim 1, wherein said junctionopenings are in a common plane.
 14. Friction vacuum pump as claimed inclaim 13, characterized in that the diameter of succeeding pump stages(13, 14) is greater than the diameter of preceding pump stages (12, 13)and that the inclination of the plane of the junction openings (36, 37)with respect to the direction of axis (15) of the rotor (4) is adaptedto the increase in the diameter.
 15. A friction vacuum pump as set forthin claim 13, wherein said common plane is disposed parallel to the axisof the rotor.
 16. A friction pump as set forth in claim 13, wherein saidcommon plane is disposed at an angle from a plane which is parallel tothe axis of the rotor.